Pretreatment of Actinidia deliciosa plants with the essential oil of Monarda didyma: effect on the disease caused by Pseudomonas syringae pv. actinidiae

Plant pathogenic bacteria have recently caused severe outbreaks in economically important crops, such as Pseudomonas syringae pv. actinidiae (Psa), which causes the bacterial canker of kiwifruit. Since the control measures of bacterial diseases are essentially preventive, the use of natural substances, such as essential oils (EOs), is becoming widespread. In 2013, the antimicrobial effectiveness of the EO from Monarda spp. against Psa was shown in vitro. Based on these findings, it was hypothesized that pretreatment of Actinidia deliciosa plants with the EO from M. didyma flowering plants might subsequently interfere with or inhibit the Psa growth. On 25 May 2015, 150 micropropagated plants were subdivided into 4 groups corresponding to 4 treatments (sterile distilled water, EO 0.3%, hydrolate 14.7%, and streptomycin 100 mg/l) and after 24h they were inoculated with a bacterial suspension of the virulent Psa IPVBO 8101 strain (3 x 107 CFU/ml). The plant health was daily checked for damages caused by each treatment itself, and for the symptoms induced by Psa. At the end of July, all leaves were collected to determine the number of spots/leaf/plant for statistical analysis of the data. The effectiveness of the EO (0.3 %) and of its hydrolate (14.7 %) was correlated with the main chemical components (i.e. thymol, p-cymene). From the results obtained in this first trial in planta, the possibility of using in the field the M. didyma EO to control the bacterial canker of kiwifruit, is discussed.